Communication control device

ABSTRACT

A communication control device of an embodiment includes a converter and a controller. The converter is input with a second digital data signal prior to input of a first digital data signal and outputs an analog data signal which is an analog signal converted into from the second digital data signal. The first digital data signal is output to a destination device. The controller invalidates the first digital data signal when a voltage corresponding to the analog data signal does not coincide with a predetermined output permission voltage and validates the first digital data signal when the voltage corresponding to the analog data signal coincides with the output permission voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is national stage application of InternationalApplication No. PCT/JP2014/060650, filed Apr. 14, 2014, which designatesthe United States, incorporated herein by reference, and which claimsthe benefit of priority from Japanese Application No. 2013-258594, filedon Dec. 13, 2013, the entire contents of which are incorporated hereinby reference.

FIELD

An embodiment of described herein relates generally to a communicationcontrol device.

BACKGROUND

Some turbine monitoring instruments used for measuring vibration,eccentricity, elongation, or elongation difference of a rotor, casing,or the like of turbine generators or electric pumps for nuclear orthermal power generation or private power generation employ acybersecurity function. Generally, a turbine monitoring instrumentmeasures vibration of a revolving shaft or a casing in a turbinegenerator or electric pump or elongation or elongation difference of therevolving shaft in the turbine generator or electric pump, or the like.Moreover, the turbine monitoring instrument transfers, to a host controldevice such as a distributed control system (DCS) or electric hydrauliccontrol (EHC) disposed in a remote place, information related to ameasurement result (e.g. measurement value or alarm) of vibration of therevolving shaft or casing in the turbine generator or electric pump orelongation or elongation difference of the revolving shaft in theturbine generator or electric pump, or the like.

Some of cybersecurity functions employed to the turbine monitoringinstruments ensure security by permitting change of setting values onlywhen a password is input to a personal computer (PC) that can change thesetting values having been set to the turbine monitoring instrument(monitor) and the input password is correct.

Alternatively, some of cybersecurity functions employed to turbinemonitoring instruments ensure security by providing a hardware key forsetting setting values or the like to the turbine monitoring instrumentand thereby changing setting values only when an operator operates thehardware key for settings to cause the turbine monitoring instrument tobe switched over to a RUN state, where the setting value or the like canbe changed.

Furthermore, in recent years, a host control device, storing datadigitalized from the measurement result or the like by the turbinemonitoring instrument, monitors at all times the turbine monitoringinstrument connected thereto via a network. In such circumstances,especially in a nuclear power plant or the like, important devices (suchas turbine monitoring instruments) are targeted in cyber-attacks morefrequently.

With a cybersecurity function using a password, however, once thepassword is solved, setting values having been set to a turbinemonitoring instrument may be easily changed and thus a monitoring targetof the turbine monitoring instrument may fall into chaos. Alternatively,a cybersecurity function using a hardware key for settings requires keyoperation to cause the RUN state with the hardware key for settingsevery time the setting values or the like having been set to the turbinemonitoring instrument are changed and management or the like is requiredfor the hardware key for settings to prevent losing or the like of thehardware key for settings. Moreover, when a spare key is produced, itbecomes difficult that the hardware key for settings plays a role as thecybersecurity function.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of acybersecurity system according to a first embodiment;

FIG. 2 is a diagram illustrating hardware configurations of first andsecond reception control units included in a communication controldevice according to the first embodiment;

FIG. 3 is a diagram for explaining reception processing of setting databy the communication control device according to the first embodiment;

FIG. 4 is a diagram illustrating hardware configurations of first andsecond reception control units included in a communication controldevice according to a second embodiment;

FIG. 5 is a diagram illustrating hardware configurations of first andsecond reception control units included in a communication controldevice according to a third embodiment; and

FIG. 6 is a diagram illustrating hardware configurations of first andsecond reception control units included in a communication controldevice according to a fourth embodiment.

In general, an according to an embodiment, a communication controldevice is provided with a converter and a controller. The converter isinput with a second digital data signal prior to input of a firstdigital data signal and outputs an analog data signal which is an analogsignal converted into from the second digital data signal. The firstdigital data signal is output to a destination device. The controllerinvalidates the first digital data signal when a voltage correspondingto the analog data signal does not coincide with a predetermined outputpermission voltage and validates the first digital data signal when thevoltage corresponding to the analog data signal coincides with theoutput permission voltage.

DETAILED DESCRIPTION

A cybersecurity system employing a communication control deviceaccording to the present embodiment will be described below with theaccompanying drawings.

First Embodiment

FIG. 1 is a block diagram illustrating a configuration of acybersecurity system according to a first embodiment. The cybersecuritysystem according to the present embodiment includes, as illustrated inFIG. 1, a monitoring device 1 for monitoring a monitoring target devicesuch as a turbine generator or an electric pump for nuclear or thermalpower generation or private power generation, a personal computer (PC) 2for transmitting, to the monitoring device 1, various data such assetting data used for changing various settings having been set to themonitoring device 1 or an alarm value used for alarm notification, and ahost controller 3 (e.g. distributed control system (DCS) or electrichydraulic control (EHC)) connected to the monitoring device 1 via a hostnetwork such as the Internet and capable of transmitting various datasuch as setting data for the monitoring device 1. In the presentembodiment, the monitoring device 1 and the PC 2 are connected with eachother via a communication cable 4 and capable of communicating inaccordance with a communication standard such as Ethernet (registeredtrademark) or RS2320.

The monitoring device 1 includes, as illustrated in FIG. 1, monitors 10and 11 including a turbine monitoring instrument or the like formeasuring vibration, eccentricity, elongation, elongation difference,revolution, etc. of a rotor, casing, or the like included in amonitoring target device, a power source device 12 for supplying powerto the monitors 10 and 11, a first reception control unit 13 forreceiving a data signal of setting data or the like from an externaldevice (such as the PC 2 and host control device 3 in the presentembodiment), a second reception control unit 14 which is connected tothe first reception control unit 13 via a communication cable 18 such asa two-line cable, which supplies power to the first reception controlunit 13, and which performs data analysis of the data signal receivedfrom the first reception control unit 13, a first transmission controlunit 15 for transmitting measurement data showing a measurement resultby the monitors 10 and 11 to the external device, and a secondtransmission control unit 16 which is connected to the firsttransmission control unit 15 via a communication cable 19 such as atwo-line cable, which supplies power to the first transmission controlunit 15, and which controls transmission of the measurement data by thefirst transmission control unit 15 to the external device.

The first and second reception control units 13 and 14 and the first andsecond transmission control units 15 and 16 are connected between themonitoring device 1 (exemplary destination device) and the externaldevice (source device) and thereby function as a communication controldevice 17 for controlling communication of the monitoring device 1 withthe external device. In the present embodiment, the communicationcontrol device 17 is included in the monitoring device 1; however, thecommunication control device 17 is not limited to be disposed thereinand may be disposed within a communication path between the monitoringdevice 1 and the external device and may be disposed, for example,outside the monitoring device 1.

In the present embodiment, the first reception control unit 13 isattachable to and detachable from the second reception control unit 14.Moreover, the first reception control unit 13 is attachable to theexternal device which is different from a device (in the presentembodiment, the monitoring device 1) mounted with the second receptioncontrol unit 14. This allows the first reception control unit 13 to bedisposed remotely from the second reception control unit 14 via acommunication cable.

In the present embodiment, the first transmission control unit 15 isattachable to and detachable from the monitoring device 1. Moreover, thefirst transmission control unit 15 is attachable to the external devicewhich is different from the device (in the present embodiment, themonitoring device 1) mounted with the second transmission control unit16. This allows the first transmission control unit 15 to be disposedremotely from the second transmission control unit 16 via acommunication cable.

When various settings or an alarm value having been set to themonitoring device 1 is changed by the external device (PC 2 or hostcontrol device 3), the communication control device 17 is input with,from the external device, an output limitation releasing signal(exemplary second digital data signal) for releasing limitation onoutput of a data signal to the monitoring device 1 prior to reception(input) of a data signal (exemplary first digital data signal) ofsetting data to be output to the monitoring device 1.

Thereafter, the communication control device 17 converts the inputoutput limitation releasing signal into an analog signal (hereinafterreferred to as analog data signal). Next, the communication controldevice 17 validates the data signal to be output to the monitoringdevice 1 when the analog data signal satisfies a predetermined condition(when a voltage corresponding to the analog data signal coincides with apredetermined output permission voltage). On the other hand, thecommunication control device 17 invalidates the data signal to be outputto the monitoring device 1 when the analog data signal does not satisfythe predetermined condition (when the voltage corresponding to theanalog data signal does not coincide with the predetermined outputpermission voltage). When the data signal to be output to the monitoringdevice 1 is invalidated, and thereafter if the output limitationreleasing signal is then repeatedly input, and the analog data signalconverted into from the output limitation releasing signal having beeninput does not satisfy the predetermined condition successively for apredetermined number of times (e.g. twice), the communication controldevice 17 transmits alarm information to the external device andprohibits input of the output limitation releasing signal.

This allows for invalidating setting data for the monitoring device 1 byinvalid access to the monitoring device 1 and thereby preventing thatimportant information such as various settings or an alarm value havingbeen set to the monitoring device 1 from being overwritten when thepredetermined output permission voltage to be compared to the voltagecorresponding to the analog data signal is not solved. Therefore,security of the monitoring device 1 can be ensured while a systemincluding the monitoring device 1 (e.g. power generation system such asa nuclear power plant) can be prevented from falling into chaos.

Next, specific configurations of the first and second reception controlunits 13 and 14 in the communication control device 17 included in thecybersecurity system according to the present embodiment will bedescribed with reference to FIG. 2. FIG. 2 is a diagram illustratinghardware configurations of the first and second reception control unitsincluded in the communication control device according to the firstembodiment.

In the present embodiment, the first reception control unit 13 includes,as illustrated in FIG. 2, a capacitor 131 charged by the outputlimitation releasing signal (or data signal) input from the externaldevice via the communication cable 4, a rectifying element (diode) 132for preventing backflow of a current to the external device from thecapacitor 131, and a bipolar transistor 133 where a charging voltage ofthe capacitor 131 is applied to a base thereof. In the presentembodiment, as illustrated in FIG. 2, the first transmission controlunit 15 transmits, to the external device via the communication cable 4,a digital signal (a signal to be transmitted by the monitoring device 1to the external device such as a data signal of a measurement result ofvibration, eccentricity, elongation, elongation difference, revolution,etc. of a rotor, casing, or the like included in a monitoring targetdevice) modulated by a modulation unit 151 included in the firsttransmission control unit 15.

In the present embodiment, a collector of the bipolar transistor 133 isconnected to a battery 141 while an emitter thereof is connected to anA/D conversion circuit 143 and a demodulator 145. The battery 141, A/Dconversion circuit 143, and demodulator 145 will be described later.Also, the bipolar transistor 133 outputs a current I corresponding tothe charging voltage applied to the base thereof by the capacitor 131.As a result, the capacitor 131 and bipolar transistor 133 convert thevoltage, corresponding to the analog data signal which is an analogsignal having been converted into from the output limitation releasingsignal, into a current and thereby outputs the current. In the presentembodiment, therefore, the first reception control unit 13 functions asa converter that converts the output limitation releasing signal inputfrom the external device into the analog data signal and thereby outputsthe signal. Also, the capacitor 131 and bipolar transistor 133 convertsthe voltage, corresponding to the analog signal having been convertedinto from the data signal, into a current and thereby outputs thecurrent.

Here, the reason for converting the voltage corresponding to the analogsignal having been converted into from the output limitation releasingsignal (or data signal) into the current I in the first receptioncontrol unit 13 is to prevent that, when each of the first receptioncontrol unit 13 and second reception control unit 14 is mounted toseparate devices, the analog signal having been converted into from theoutput limitation releasing signal (or data signal) cannot be output tothe second reception control unit 14 with a high accuracy due to avoltage drop of the voltage corresponding to the analog signal havingbeen converted into from the output limitation releasing signal (or datasignal) due to a resistance in a communication cable connecting thefirst reception control unit 13 and second reception control unit 14.

The second reception control unit 14 includes, as illustrated in FIG. 2,the battery 141 connected to the collector of the bipolar transistor 133included in the first reception control unit 13, an I/V conversion loadresistance 142 for clarifying a logic level of the digital signalconverted into from the analog data signal by the A/D conversion circuit143, which will be described later, the A/D conversion circuit 143 forconverting into a voltage based on the current I output from the bipolartransistor 133 (in other words, the A/D conversion circuit 143 forconverting the analog data signal output from the bipolar transistor 133in the first reception control unit 13 into a digital signal), a voltagecomparison circuit 144 for invalidating a data signal input from theexternal device when the voltage (voltage corresponding to the digitalsignal converted into from the analog data signal) converted into by theA/D conversion circuit 143 does not coincide with the predeterminedoutput permission voltage and validating the data signal input from theexternal device when the voltage converted into by the A/D conversioncircuit 143 coincides with the predetermined output permission voltage,and the demodulator 145 for extracting (demodulating) the data signalfrom the current I (current I converted into from the voltagecorresponding to the analog signal of the data signal) output from thebipolar transistor 133, and a backflow prevention diode 146 forpreventing backflow of a signal (signal to be transmitted from themonitoring device 1 to the external device) input to the communicationcontrol device 17 from the monitoring device 1 to the second receptioncontrol unit 14.

In the present embodiment, the second reception control unit 14functions as a controller which invalidates the data signal when thevoltage corresponding to the analog data signal does not coincide withthe predetermined output permission voltage and validates the datasignal when the voltage corresponding to the analog data signalcoincides with the predetermined output permission voltage. Here, thecase where the voltage corresponding to the analog data signal coincideswith the predetermined output permission voltage includes a case wherethe voltage corresponding to the analog data signal is within apredetermined voltage range in relation to the output permissionvoltage. Also, in the present embodiment, the communication controldevice 17 outputs, to the monitoring device 1, the output limitationreleasing signal and the data signal input from the external deviceusing the same communication path and thus the data signal is alsoconverted into an analog signal once and then demodulated into thedigital signal again for output in a similar manner to that for theoutput limitation releasing signal. However, the data signal may beoutput to the monitoring device 1 without being converted into an analogsignal by providing a communication path dedicated to the data signal.

Next, reception processing of setting data by the communication controldevice 17 according to the present embodiment will be described withreference to FIGS. 2 and 3. FIG. 3 is a diagram for explaining receptionprocessing of setting data by the communication control device accordingto the first embodiment.

In the cybersecurity system according to the present embodiment, asillustrated in FIG. 3, an output limitation releasing signal C is inputto the monitoring device 1 prior to input of a data signal D to themonitoring device 1 when the data signal D of setting data is input tothe monitoring device 1 from the external device. When the outputlimitation releasing signal C is input, charging of the capacitor 131 inthe first reception control unit 13 is initiated. When charging of thecapacitor 131 is initiated, the bipolar transistor 133 is applied withthe charging voltage of the capacitor 131 from the base thereof andoutputs, to the second reception control unit 14, the current Icorresponding to the charging voltage of the capacitor 131.

The A/D conversion circuit 143 in the second reception control unit 14converts the current I output from the bipolar transistor 133 into avoltage V. The current I output from the bipolar transistor 133increases as the charging voltage of the capacitor 131 increases andthus the voltage V output from the A/D conversion circuit 143 increasesaccordingly. As illustrated in FIG. 3, the voltage comparison circuit144 validates the data signal D (data signal D output from thedemodulator 145) input after the output limitation releasing signal Cwhen the current I output from the bipolar transistor 133 increases andthereby the voltage V at time t1 output from the A/D conversion circuit143 coincides with a predetermined output permission voltage Vth.

In the present embodiment, the voltage comparison circuit 144 commandsthe monitoring device 1 to execute processing corresponding to the datasignal D output from the demodulator 145 and thereby validates the datasignal D output from the demodulator 145. For example, when a resistancevalue of the I/V conversion load resistance 142 is 250 Ω and thepredetermined output permission voltage Vth is 3.0 V, the voltagecomparison circuit 144 validates the data signal D output from thedemodulator 145 when the current I output from the bipolar transistor133 equals 12 mA while the voltage V output from the A/D conversioncircuit 143 equals 3.0 V since the voltage V output from the A/Dconversion circuit 143 coincides with the predetermined outputpermission voltage Vth.

On the other hand, the voltage comparison circuit 144 invalidates thedata signal D output from the demodulator 145 when the voltage V(voltage V output from the A/D conversion circuit 143) based on thecurrent I output from the bipolar transistor 133 does not coincide withthe predetermined output permission voltage Vth (specifically, when thevoltage V does not reach the predetermined output permission voltage Vthor the voltage V exceeds the predetermined output permission voltageVth). In the present embodiment, the voltage comparison circuit 144commands the monitoring device 1 to prohibit processing corresponding tothe data signal D output from the demodulator 145 and therebyinvalidates the data signal D output from the demodulator 145.

Meanwhile, the current I output from the bipolar transistor 133 variesaccording to the charging voltage of the capacitor 131 as describedabove. The charging voltage of the capacitor 131 further variesaccording to a duty ratio of the output limitation releasing signal Cinput from the external device. That is, the current I output from thebipolar transistor 133 is dependent on the duty ratio of the outputlimitation releasing signal C input from the external device.

Therefore, in the communication control device 17 according to thepresent embodiment, even when a content of data shown by the outputlimitation releasing signal C is different, the data signal D isvalidated when the duty ratio of the output limitation releasing signalC input from the external device is such that the voltage V based on thecurrent I output from the bipolar transistor 133 equals thepredetermined output permission voltage Vth. This allows for mitigatingpossibilities that the data signal D input by an invalid access from theexternal device is validated as long as such a duty ratio that thevoltage V based on the current I output from the bipolar transistor 133equals the predetermined output permission voltage Vth is not solved byvalidating the data signal D using the output limitation releasingsignal C with data a content of which is different each time the datasignal D is input from the external device. Thus, security of themonitoring device 1 can be ensured.

Also, in the present embodiment, the voltage comparison circuit 144validates the data signal D output from the demodulator 145 when thevoltage V output from the A/D conversion circuit 143 coincides with thepredetermined output permission voltage Vth continuously for apredetermined period of time (as illustrated in FIG. 3, when the voltageV equals to the predetermined output permission voltage Vth at time t1and continues to be equal to the predetermined output permission voltageVth until time t2, when a predetermined period of time has passed). Thisprevents validation of the data signal D output from the demodulator 145when the voltage V output from the A/D conversion circuit 143temporarily equals to the predetermined output permission voltage Vth byinput of a signal having a duty ratio larger than that of the outputlimitation releasing signal C that allows the A/D conversion circuit 143to output the voltage V equal to the predetermined output permissionvoltage Vth.

In this manner, according to the communication control device 17 of thefirst embodiment, possibility that, the data signal D input by aninvalid access from the external device is validated, can be reduced,thereby allowing for ensuring security of the monitoring device 1.

Second Embodiment

The present embodiment is an example where output of a data signal to amonitoring device 1 is prohibited and thereby the data signal isinvalidated when a voltage corresponding to an analog data signal havingbeen converted into from an output limitation releasing signal does notcoincide with a predetermined output permission voltage and output ofthe data signal to the monitoring device 1 is permitted and thereby thedata signal is validated when the voltage corresponding to the analogdata signal having been converted into from the output limitationreleasing signal coincides with the predetermined output permissionvoltage. In the descriptions below, description on a configurationsimilar to that of the first embodiment will be omitted.

FIG. 4 is a diagram illustrating hardware configurations of first andsecond reception control units included in a communication controldevice according to a second embodiment. A second reception control unit401 in a communication control device 400 according to the presentembodiment includes, as illustrated in FIG. 4, a battery 141, an I/Vconversion load resistance 142, an A/D conversion circuit 143, ademodulator 145, a backflow prevention diode 146, and an outputcontroller 402 for prohibiting output of the data signal input from theexternal device to the monitoring device 1 when the voltage having beenconverted into by the A/D conversion circuit 143 (voltage correspondingto a digital signal having been converted into from the analog datasignal) does not coincide with the predetermined output permissionvoltage and permitting output of the data signal input from the externaldevice to the monitoring device 1 when the voltage having been convertedinto by the A/D conversion circuit 143 coincides with the predeterminedoutput permission voltage.

In the present embodiment, the output controller 402 includes a switchunit 404 capable of prohibiting or permitting output of the data signalextracted by the demodulator 145 to the monitoring device 1, and avoltage comparison circuit 403 for prohibiting output of the data signalby turning off the switch unit 404 when the voltage having beenconverted into by the A/D conversion circuit 143 does not coincide withthe predetermined output permission voltage and permitting output of thedata signal by turning on the switch unit 404 when the voltage havingbeen converted into by the A/D conversion circuit 143 coincides with thepredetermined output permission voltage.

In this manner, the communication control device 400 of the secondembodiment can provide similar effects to those of the first embodiment.

Third Embodiment

The present embodiment is an example where the first reception controlunit includes a D/A converter for converting an output limitationreleasing signal into an analog data signal and a bipolar transistor, tobe applied with a voltage corresponding to the analog data signal havingbeen converted into by the D/A converter from a base thereof, foroutputting a current corresponding to the voltage. In the descriptionsbelow, description on a configuration similar to that of the firstembodiment will be omitted.

FIG. 5 is a diagram illustrating hardware configurations of first andsecond reception control units included in a communication controldevice according to a third embodiment. A first reception control unit501 in a communication control device 500 according to the presentembodiment includes, as illustrated in FIG. 5, a D/A conversion circuit502 for converting the output limitation releasing signal input from anexternal device via a communication cable 4 into an analog data signaland a bipolar transistor 503, to be applied with a voltage correspondingto the analog data signal having been converted into by the D/Aconversion circuit 502 from a base thereof, for outputting a current Icorresponding to the voltage. Moreover, in the present embodiment, theD/A conversion circuit 502 converts a data signal input from theexternal device into an analog signal. Furthermore, the bipolartransistor 503 is applied with a voltage corresponding to the analogsignal having been converted into from the data signal by the D/Aconversion circuit 502 from abase thereof and outputs the current Icorresponding to the voltage. In this case, security of the monitoringdevice 1 can be enhanced by changing a predetermined output permissionvoltage as appropriate.

In this manner, according to the communication control device 500 of thethird embodiment, when the predetermined output permission voltage to becompared to the voltage corresponding to the analog data signal havingbeen converted into from the output limitation releasing signal by theD/A conversion circuit 502 is not solved, output of setting data for themonitoring device 1 by invalid access to the monitoring device 1 can beprevented, thereby preventing that important information such assettings or an alarm value having been set to the monitoring device 1 isoverwritten. Therefore, security of the monitoring device 1 can beensured while a system including the monitoring device 1 can beprevented from falling into chaos.

Fourth Embodiment

The present embodiment is an example where output of a data signal to amonitoring device 1 is prohibited and thereby the data signal isinvalidated when a voltage corresponding to an analog data signal havingbeen converted into from an output limitation releasing signal does notcoincide with a predetermined output permission voltage and output ofthe data signal to the monitoring device 1 is permitted and thereby thedata signal is validated when the voltage corresponding to the analogdata signal having been converted into from the output limitationreleasing signal coincides with the predetermined output permissionvoltage. In the descriptions below, descriptions on a configurationsimilar to that of the second or third embodiment will be omitted.

FIG. 6 is a diagram illustrating hardware configurations of first andsecond reception control units included in a communication controldevice according to a fourth embodiment. A communication control device600 according to the present embodiment includes the first receptioncontrol unit 501 of the third embodiment and the second receptioncontrol unit 401 of the second embodiment.

In this manner, the communication control device 600 of the fourthembodiment can provide similar effects to those of the third embodiment.

As described above, according to the first to fourth embodiments,security of the monitoring device 1 can be ensured while a systemincluding the monitoring device 1 can be prevented from falling intochaos.

Some embodiments of the present invention have been described; however,these embodiments have been proposed as examples and are not intended tolimit the scope of the invention. These novel embodiments can be carriedout in other various embodiments and may include various omissions,replacement, or variation within a scope not departing from theprincipals of the invention. These embodiments or variations thereof areincluded in the scope or principals of the invention and also includedin a scope equivalent to aspects of the invention described in theclaims.

What is claimed is:
 1. A communication control device comprising: aconverter configured to be input with a second digital data signal priorto input of a first digital data signal and to output an analog datasignal which is an analog signal converted from the second digital datasignal, the first digital data signal being output to a destinationdevice; and a controller configured to invalidate the first digital datasignal when a voltage corresponding to the analog data signal does notcoincide with a predetermined output permission voltage and to validatethe first digital data signal when the voltage corresponding to theanalog data signal coincides with the predetermined output permissionvoltage.
 2. The communication control device according to claim 1,wherein the controller prohibits output of the first digital data signalto the destination device when invalidating the first digital datasignal and permits output of the first digital data signal to thedestination device when validating the first digital data signal.
 3. Thecommunication control device according to claim 1, wherein the converterconverts the voltage corresponding to the analog data signal into acurrent and thereby outputs the current, and the controller invalidatesthe first digital data signal when a voltage based on the current outputfrom the converter does not coincide with the predetermined outputpermission voltage and to validate the first digital data signal whenthe voltage based on the current output from the converter coincideswith the predetermined output permission voltage.
 4. The communicationcontrol device according to claim 3, wherein the converter includes acapacitor configured to be charged by the second digital data signal anda bipolar transistor configured to be applied with a charging voltage ofthe capacitor from a base thereof and to output a current correspondingto the charging voltage.
 5. The communication control device accordingto claim 3, wherein the converter comprises a D/A converter configuredto convert the second digital data signal into the analog data signaland a bipolar transistor configured to be applied with the voltagecorresponding to the analog data signal converted into by the D/Aconverter from a base thereof and to output a current corresponding tothe voltage.
 6. The communication control device according to claim 1,wherein the controller validates the first digital data signal when thevoltage corresponding to the analog data signal coincides with thepredetermined output permission voltage continuously for a predeterminedperiod of time.
 7. The communication control device according to claim3, wherein, the communication control device includes a device mountedwith the controller and an external device which is different from thedevice, the converter is mounted to the external device, and the deviceis connected to the external device via a communication cable.